Evidence for the coincident initiation of homolog pairing and synapsis during the telomere-clustering (bouquet) stage of meiotic prophase

Telomere associations in interphase nuclei: possible role in maintenance of interphase chromosome topology

The relative sizes of individual telomeres in cultured human cells under conditions of cell cycling, replicative quiescence, cell transformation and immortalization were determined using quantitative fluorescence in situ hybridization (Q-FISH) with a telomere-specific peptide nucleic acid (PNA) probe. Results obtained from analysis of telomere length profiles (TLPs), which display the distribution of relative telomere lengths for individual cells, confirmed telomere length heterogeneity at the single cell level and proportional shortening of telomere length during replicative aging of virus-transformed cells. TLPs also revealed that some telomeric ends of chromosomes are so closely juxtaposed within interphase nuclei that their fluorescent signals appear as a single spot. These telomeric associations (TAs) were far more prevalent in interphase nuclei of noncycling normal and virus-transformed cells than in their cycling counterparts. The number of interphase TAs per nucleus observed in late-passage E6/E7-transformed cells did not increase during progression to crisis, suggesting that telomere shortening does not increase the frequency of interphase TAs. Furthermore, interphase TAs were rarely observed in rapidly cycling, telomerase-positive, immortalized cells that exhibit somewhat shortened, but stabilized, telomere length through the activity of telomerase. Our overall results suggest that the number of interphase TAs is dependent more on whether or not cells are cycling than on telomere length, with TAs being most prominent in the nuclei of replicatively quiescent cells in which nonrandom (even preferred) chromosome spatial arrangements have been observed. We propose that interphase TAs may play a role in the generation and/or maintenance of nuclear architecture and chromosome positional stability in interphase nuclei, especially in cells with a prolonged G(1)/G(0) phase and possibly in terminally differentiated cells.

A maize chromosome 3 addition line of oat exhibits expression of the maize homeobox gene liguleless3 and alteration of cell fates

Maize chromosome addition lines of oat offer the opportunity to study maize gene expression in oat and the resulting phenotypes. Morphological examination of a maize chromosome 3 addition line of oat showed that this line exhibited several morphological abnormalities including a blade-to-sheath transformation at the midrib region of the leaf, a hook-shaped panicle, and abnormal outgrowth of aerial axillary buds. Dominant mutations in the maize liguleless3 (lg3) homeobox gene result in a blade (distal)-to-sheath (proximal) transformation at the midrib region of the leaf. Ectopic expression of the dominant mutant Lg3 allele is believed to cause the phenotype. Therefore, we suspected that the maize lg3 gene, which is located on maize chromosome 3, was involved in the phenotypes observed in the maize chromosome 3 addition line of oat. Genetic analyses of an oat BC1F2 family segregating for maize chromosome 3 showed that the presence of a stable maize chromosome 3 was required for the expression of these cell fate abnormalities. RNA expression analysis of leaf sheath tissue from oat plants carrying maize chromosome 3 demonstrated that maize LG3 transcripts accumulated in oat, indicating that this expression is associated with the blade-to-sheath transformation, hook-shaped panicle and outgrowth of aerial axillary bud phenotypes. Our results demonstrate that the maize chromosome addition lines of oat are useful genetic stocks to study expression of maize genes in oat.Key words: liguleless3, homeobox, oat-maize addition line.

Construction and transposition of a 100-kilobase extended P element in Drosophila

We have used P element deletion derivatives at defined locations in the Drosophila genome to construct a 100-kb extended P element more than twice the size of any previously available. We demonstrate that this prototypical extended P element is capable of transposition to new sites in the genome. The structural and functional integrity of a transposed extended P element was confirmed using molecular, genetic, and cytogenetic criteria. This is the first method shown to be capable of producing large, unlinked transpositional duplications in Drosophila. The ability to produce functional transposable elements from half-elements is novel and has many potential applications for the functional analysis of complex genomes.

Production and characterization of maize chromosome 9 radiation hybrids derived from an oat-maize addition line

In maize (Zea mays L., 2n = 2x = 20), map-based cloning and genome organization studies are often complicated because of the complexity of the genome. Maize chromosome addition lines of hexaploid cultivated oat (Avena sativa L., 2n = 6x = 42), where maize chromosomes can be individually manipulated, represent unique materials for maize genome analysis. Maize chromosome addition lines are particularly suitable for the dissection of a single maize chromosome using radiation because cultivated oat is an allohexaploid in which multiple copies of the oat basic genome provide buffering to chromosomal aberrations and other mutations. Irradiation (gamma rays at 30, 40, and 50 krad) of a monosomic maize chromosome 9 addition line produced maize chromosome 9 radiation hybrids (M9RHs)-oat lines possessing different fragments of maize chromosome 9 including intergenomic translocations and modified maize addition chromosomes with internal and terminal deletions. M9RHs with 1 to 10 radiation-induced breaks per chromosome were identified. We estimated that a panel of 100 informative M9RHs (with an average of 3 breaks per chromosome) would allow mapping at the 0. 5- to 1.0-Mb level of resolution. Because mapping with maize chromosome addition lines and radiation hybrid derivatives involves assays for the presence or absence of a given marker, monomorphic markers can be quickly and efficiently mapped to a chromosome region. Radiation hybrid derivatives also represent sources of region-specific DNA for cloning of genes or DNA markers.